1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as, for example, an electrophotographic copying machine or an electrophotographic printer, of a type employing a replaceable image forming unit or cartridge including a photosensitive drum and a developing device.
2. Description of the Related Art
An image forming apparatus, for example, a printer, of a type wherein a image forming cartridge including a photo-sensitive drum and a developing device is replaceable relative to the body of the printer is well known such as disclosed in, for example, U.S. Pat. No. 4,791,454 issued Dec. 13, 1988, and U.S. Pat. No. 5,036,358 issued Jul. 30, 1991, and the Japanese Laid-open Patent Publications No. 1-316765 published Dec. 21, 1989, and No. 1-319065 published Dec. 25, 1989.
Of those prior art references, the Japanese Laid-open Patent Publication No. 1-319065 discloses a system for detecting whether or not the replaceable image forming cartridge is installed inside the image forming apparatus and also the amount of a developing material contained in a developer container. This known system comprises windows formed in a wall, defining a developer container, a transparent windowpane covering each of the windows, a stirrer blade assembly rotatably supported within the developer container and having cleaning members that sweeps the windowpanes during the rotation of the stirrer blade assembly while stirring the developing material, a light emitting element for emitting rays of light and a light receiving element for receiving the rays of light emitted by the light emitting element. In this known system, while the light emitting and receiving elements are installed in the body of the printer, the rays of light emitted by the light emitting element are adapted to enter the developer chamber through one of the transparent windowpanes and then emerge outwardly from the developer chamber through the otherof the windowpanes towards the light receiving element.
More specifically, the detecting system disclosed in the publication No. 1-319065 is such that, when the image forming apparatus is electrically powered on, a signal necessary to drive the stirrer blade assembly in the image forming unit is outputted and, at the same time, the light emitting element is electrically energized to emit rays of light. The rays of light emitted from the light emitting element travel towards the light receiving element across a portion of the housing, forming the replaceable image forming unit, by way of the transparent windowpanes.
The light receiving element provides an output which is classified into of one of three different patterns depending on particulars of the image forming cartridge, that is, the presence or absence of the image forming cartridge within the image forming apparatus and/or the presence or absence of the developing material within the hopper in the image forming cartridge. Specifically, when the light receiving element receives the incoming light at all times without being intercepted by the stirrer blade assembly, the output from the light receiving element represents a first pattern descriptive of the presence of the image forming cartridge inside the image forming apparatus; when the light receiving element receives the incoming light which is cyclically intercepted by the stirrer blade assembly then rotating in one direction, the output from the light receiving element represents a second pattern descriptive of an insufficient amount of developing material contained in the hopper; and when the light receiving element receives no incoming light because the latter is intercepted by the mass of the developing material within the hopper regardless of an operating condition of the stirrer blade assembly, the output from the light receiving element represents a third pattern descriptive of a sufficient amount of developing material contained in the hopper.
However, the prior art detecting system discussed above requires a drive system for the entire image forming apparatus to be activated to drive the stirrer blade assembly regardless of the presence or absence of the image forming cartridge loaded inside the image forming apparatus. In other words, this prior art image forming apparatus is required to be unnecessarily operated for the detecting system to work for a predetermined length of time prior to an actual image forming cycle taking place.